In order to satisfy various consumer requirements, automobile lacquering nowadays takes the form of a multi-layer lacquer coating. In this connection the most diverse lacquer films serve various purposes, for example, to achieve protection against the impact of stones, to achieve corrosion protection, or to obtain a good, optically appealing surface. It is known that the primer for achieving corrosion protection can be produced from coating agents on an anionic or cationic basis.
The layers of primer surfacer which are necessary to achieve a sufficient degree of protection against the impact of stones are nowadays based either on a solvent-containing formulation or an aqueous formulation. Up to the present time the only known systems formulated on an anionic basis have been aqueous systems. These coating agents have the disadvantage that in those places where damage has occurred to the layer of anti-corrosion primer only a poor degree of protection against corrosion is afforded. Furthermore it has been shown that the stoving temperatures of the layer of primer surfacer are relatively high. Owing to practical considerations, eg. energy costs or the dimensional stability of plastic substrates, it is necessary, however, to keep the stoving temperatures of the lacquer layers as low as possible.
Binding agents based on a cationic process which are used for corrosion-protection primers are already described in the patent literature. These are deposited by electrophoretic lacquering, ie. an aqueous solution of the binding agents together with conventional additional materials is produced and this is deposited by applying an electric current to the metallic workpiece used as cathode. Then the coated substrate is stoved at temperatures between 150.degree. and 200.degree. C., ie., chemical crosslinking of the coating takes place. Examples of such coating agents are described in DE-OS 36 34 483, DE-OS 36 14 551, DE-OS 36 14 435, EP-A 54 193 and EP-A 193 103. Use is made of binding agents for lacquers which are capable of being deposited at the cathode (KTL) based on amino acrylate resins, amino epoxide resins or amino urethane resins. These are mixed with pigments in a pigment/binding-agent ratio of up to 0.5:1 and dispersed, and with the addition of conventional lacquer additives the coating agent is produced. The solids content of the coating agents generally amounts to 12-22% by weight. Following precipitation these coating agents are stoved at temperatures exceeding 150.degree. C.
These coating agents have the disadvantage that they only contain a small proportion of solids and are therefore not suitable for application by spray. So it is first necessary, by means of stoving, to bring about evaporation of the water still present in the coating agent. In addition, high temperatures are required for crosslinking the coating agent, so that the choice of useable substrates is limited.
Coating agents for offering protection against the impact of stones are equally well-known. These so-called fillers are, for example, known from DE-OS 40 00 748, EP-A 249 727 or DE-OS 38 13 866. Use is made of coating agents based on anionically stabilised coating agents which are processed with conventional pigments and additives to produce the coating agent. Polyurethane resins and reaction products of polyesters and epoxide resins are described as the basis for binding agents. By way of crosslinking agents melamine resins and blocked isocyanates are described. These coating agents have the disadvantage that they require relatively high stoving temperatures of about 150.degree. C. Similarly it has been shown that bare metal parts which have no protective layer of anti-corrosion primer have inadequate protection against corrosion. Such imperfections can arise, for example, as a result of subsequent necessary processing of the car bodies, eg. grinding.